The present invention relates generally to a method of tracking time intervals of a communication signal and, more specifically, to an improved delayed lock loop (DLL) method. Although the present invention will be described with respect to Code-Division Multiple Access (CDMA) systems, which is a coding scheme that requires them to be synchronized with the coding scheme in the processing, it is applicable to other signal transmissions.
In a CDMA transmission, each station""s data stream is multiplied by binary bits, for example, pseudo noise (PN) codes. In order to decode the data stream at the receiver, the received signal must be correlated with the same PN codes. The received signal is subject to transmission delays and fading. If there are multi-path components, rake receivers are used, which correlate each of the multi-path components and recombine them. Thus, the transmission delay and fading of each rake receiver finger must be determined.
In addition to matching the appropriate PN codes, it is necessary to track the transmission delay of the components, as it may vary. Two of the predominant PN tracking configurations used are DLL and Tau-Dither Loop (TDL). Both are early-late gate-type loops where the received PN code is correlated with delayed and advanced versions of the receiver local code PN generator output to produce an error correcting characteristics.
Two typical examples are illustrated in FIGS. 1 and 2. The input signal is provided to an early and late correlator, which correlates the input sample signal to a PN code-generated signal. The output is provided through an output detector to a difference element. The difference or error is provided through a low pass filter (LPF) to a voltage control oscillator (VCO), which changes the phase of the PN generator. The output of the PN generator is the on-time PN sequence. The other example illustrated in FIG. 2 performs generally the same process, except that it adjusts the delay of the data to account for the difference or error. The input signal is provided through a variable delay, which is connected to the early correlator, the late correlator and an on-time correlator. The results are provided to accumulators, which is provided to a decision circuitry. The off-set or error is computed and provided through a loop filter to a delay adjust, which adjusts the variable delay input of the sampled, received signal or data.
These delayed lock loops or correlators are run for a fixed correlation length, and the sample position or time of the early and late correlators compared to the on-time correlators are generally fixed. This leads to a large number of calculations and consumption of power.
The present invention offers an improved tracking system, illustrated as a delayed lock loop. The time intervals of a communication signal are tracked by first sampling the communication signal. A standard and a sample, advanced relative to each other by i sample positions from an assumed sample position representing the assumed arrival position ta of a time interval, are correlated over a correlating length of k samples. Also, the standard and a sample, retarded relative to each other by the i sample positions from the assumed sample position of ta, are also correlated over k samples. A timing error is determined from the correlations. A prediction of the predicted arrival time tp; of the next interval is determined from the error. One or more of the correlation length k or the i sample position is modified based on the predicted arrival time tp. The process is repeated using any of the modified correlation length k and/or sample position i.
The method of determining the correlation error includes calculating an estimated arrival time to of the present interval. This estimated arrival time to is used to predict the next arrival time tp. The estimated time to is a function of the magnitude and the spacing of the maximum and minimum error. The prediction of the next arrival time tp includes setting the assumed arrival time ta to the previous assumed arrival time ta, if the predicted arrival time tp differs from the previous assumed arrival time ta by less than a given value. The next assumed arrival time ta is set to the previous assumed arrival time ta plus or minus an increment, if the predicted arrival time tp differs from the previous assumed arrival time to by more than a given value. Preferably, the estimated arrival times to are passed through a predictor type filter, and the results are used to predict the next arrival time tp.
The correlation length k may be in a range of 256 to 2048 samples and may be modified by a range of 32 to 128 samples. The correlation length k will be changed depending upon whether the variance of the estimated arrival time to from the assumed arrival time ta is within or without a tolerance window. The tolerance levels correspond to the AWGN noise variances.
The method also includes determining if the error is above a threshold. If it is above a threshold, then the data samples timing of standard is interpolated, and the interpolated timing is used to decode the communication signal. If it is not above the threshold, the standard timing is used to decode the communication signal. The method may be used with CDMA communication signals and may be used in rake receivers for each finger. Time intervals may be frames, slots or other time intervals of a communication signal. Preferably, the method is carried out in a software implementation on a signal processor.
These and other aspects of the present invention will become apparent from the following detailed description of the invention, when considered in conjunction with accompanying drawings.